Chlamydia trachomatis is a prokaryotic intracellular parasite, various strains of which infect epithelial cells of the eyes (chronic conjunctivitis, trachoma), the genital tract, and the lungs. C. trachomatis infections of whole animals exhibit latency. Very little is known about the factors that control the transition from a latent to an active infection. Most of the workers who have been interested in the latency phenomenon have focused their attention on the establishment and study of latency in cell cultures. Because the ocular strains of C. trachomatis are difficult to grow in cell culture, latent cell culture infections involving these strains have not been established until recently. More recently, it has been shown that baby hamster kidney (BHK) cells grown in monolayer culture can support a latent infection by an ocular C. trachomatis strain, and that the cyclic nucleotides cAMP and cGMP appear to be involved in the maintenance of and escape from latency. This observation suggests that hormones (e.g., stress-related hormones) can trigger the recurrence of an overt infection. But in that work, the culture medium for the cells was not hormonally defined. Since it has recently become possible to grow kidney epithelial cells in hormonally defined medium, all the elements of an excellent system now exist for studying the roles of these cyclic nucleotides, and substances that affect their levels, on the maintenance of and escape from latency under fully defined culture conditions. We propose to establish monolayer cultures of baby hamster kidney (BHK) cells in a fully defined medium, and then to produce in them a latent ocular-strain C. trachomatis infection. We will then systematically examine the effects of the cyclic nucleotides cAMP and cGMP, and compounds that affect their intracellular levels, on the maintenance of and escape from latency. We will track the escape of the infecting organisms from latency quantitatively, by counting newly appearing intracellular infection sites (inclusion bodies) and by immunoassaying for the presence of shed chlamydial antigens in the culture medium above the infected cells.